Generally, the proper amount of refrigerant to be retained in a cooling system is specified according to the cooling system employed, and any shortage or overcharge of the refrigerant leads to a reduced cooling performance of the system. To avoid such shortage or overcharge of the refrigerant, various devices for detecting the amount of refrigerant have been developed, and a typical example of such prior art devices is disclosed in Japanese Utility Model Application No. 130175/80. This prior art device comprises a magnet-mounted float, a guide member adapted to guide said float and a lead switch housed in said guide member for detecting the ascent or descent of said float. In such a device, however, since the float is made of a light-weight material such as a foamed resin because the float must be kept floating in refrigerant having a small specific gravity, there have been the problems regarding the structural strength of the device and the accuracy of float movement in accordance with the change of refrigerant level in the tank.